This　study,　employed　a　novel,　patented　twin-reactor　self-recirculating　system　to　explore　the　granule　size　control　strategies　and　its　controlling　mechanisms　of　anaerobic　ammonium　oxidation　(Anammox)　granular　sludge　based　on　granules　size　recirculation　approach.　By　adjusting　internal　recirculation　locations,　the　strategy　aimed　to　control　the　size　of　granules　through　introducing　sludge　of　varying　sizes　into　regions　with　high　substrate　concentration　and　mechanical　shear.　The　result　demonstrated　that　this　strategy　effectively　increased　granule　size,　and　maintained　it　within　an　appropriate　range.　Furthermore,　it　enhanced　sludge　concentration,　settling　characteristics,　and　nitrogen　removal　performance.　Specifically,　recirculating　larger　granules　proved　to　be　highly　effective,　keeping　the　size　of　granular　sludge　within　the　optimal　range　of　0.5~1.6mm,　with　a　median　size　of　948µm,　SVI　of　35.6mL/g,　and　an　enhanced　total　inorganic　nitrogen　(TIN)　removal　efficiency　of　88.59%.　Analysis　of　extracellular　polymeric　substances　(EPS)　revealed　that　proteins　(PN)　in　the　tightly　bound　EPS　(TB-EPS)　played　a　crucial　role　in　granule　size　control.　Stratification　experiments　revealed　that　PN　in　the　outer　layer　of　the　sludge　primarily　influenced　settleability,　while　PN　in　the　TB-EPS　of　the　inner　layer　strengthens　the　granule　structure.　High-throughput　sequencing　showed　a　significant　increase　in　the　abundance　of　anammox　bacteria　(AnAOB)　Candidatus　Kuenenia,　mainly　located　in　the　inner　layers　of　the　sludge　granules.　These　findings　have　significant　implications　for　the　engineering　application　of　anammox　granular　sludge　technology.　©　2024　Chinese　Society　for　Environmental　Sciences.　All　rights　reserved.